1. Field of Invention
The invention relates generally to systems and methods to improve and extend the color gamut of a printer.
2. Description of Related Art
Printers generally have an output which can be defined as existing in a color space called CMYK, referring to the cyan, magenta, yellow, and black colorants, which is uniquely defined for the printer by its capabilities and colorants. The printer receives information in a first color space, which may have values defined in an independent color space that is not used by any device, and converts that information to print in a second color space that is dependent of device characteristics.
There are many methods for converting between color spaces, all of which begin with the measurement of the printer response to certain input values. Commonly, a printer is driven with a set of color input values, the values are printed during normal operation of the printer, and measurements are made of the printed colors to determine what the actual color was printed in response to the color specification. Most printers have non-linear response characteristics.
Calibrating a printer involves finding what set of signals must be sent to a printer to obtain a desired color. The desired color is described in some device-independent terminology, such as, for example, some well-defined standard. In contrast, the signals to the printer constitute a device-dependent terminology. A complete calibration will transform the device-independent color description into a device-dependent description such that the resultant combination of materials, such as, for example, ink, toner, dye, etc., on the paper produces the desired color, i.e., the color which was initially described in a device-independent fashion.
Calibrating high quality printers can be divided into three major tasks, (1) setting a gray balance, (2) determining black addition and under-color removal, if any; and finally (3) obtaining an apparatus color correction or color transformation. A side effect from converting from CMY to CMYK is that the gamut may be reduced. That is, the number of colors that are produced may be reduced, due to loss of hue. This side effect can be compensated for by using an under-color addition process. The under-color addition process regains lost hues and expands the gamut. A gray component replacement strategy may use both under-color removal (UCR) and under-color addition (UCA or K+).
In U.S. Pat. No. 4,500,919 to Schreiber, and U.S. Pat. No. 4,275,413 to Sakamoto each incorporated herein by reference in its entirety, the information derived from sample patch measuring was placed into look-up tables, stored in a memory, perhaps ROM memory or RAM memory, where the look-up table relates input color space to output color space. The look-up table is commonly a three-dimensional table, since color space is three dimensional. With a scanner or computer, the RGB space can be defined as three-dimensional with black at the origin of a three-dimensional coordinate system (0,0,0), and white at the maximum of a three dimensional coordinate system. In an 8-bit system, the maximum would be located at a point having coordinates of (255, 255, 255). In an RGB space each of the three axes radiating from the origin point therefore respectively define the red, green, and blue components.
A similar construct can be made for the printer, with axes representing cyan, magenta, and yellow. Black is usually a separate toner which is added separately. In the 8-bit system suggested above there will be, however, over 16 million possible colors (2563). There are clearly too many values for a 1:1 mapping of RGB colors to CMYK colors. Accordingly, as proposed in U.S. Pat. No. 4,275,413 to Sakamoto, only a relatively small number of samples are made at the printer, perhaps on the order of 1,000 samples, or even fewer. Therefore, the look-up tables include a set of values which could be said to be the intersections for comers of a set of rectangular parallelepipeds mounted on top of one another. Colors falling within each rectangular volume can be interpolated from the measured values, through a variety of methods, including tri-linear interpolation, tetrahedral interpolation, polynomial interpolation, linear interpolation, and any other appropriate interpolation method depending on the accuracy of the desired result.
An example of a method involved in such printers is found in U.S. Pat. No. 5,710,824, which discloses a method for printing in a color printer so that scanned color images defined in terms of calorimetric color signals may be printed on a color printer responsive to printer colorant signals to render a color print with a set of three primary colorants and black on a substrate. The method includes scanning an image to derive a set of device-independent colorimetric color signals. Then, the colorimetric color signals are converted into device-dependent primary colorant signals. Each primary colorant signal defines a density of colorant to be used in rendering a color print. The conversion accounts for a subsequent black colorant addition. Next, a black colorant signal is determined as a function of minimum and maximum values of the combination of primary colorant signals. The determined black color signal adds black colorant as a nonlinear function of the primary colorant signals to expand the printable color gamut. Then, the primary colorant signals are gray balanced and black is linear zed to generate a set of corresponding printer colorant signals to control the printer. Finally, the printer colorant signals are used to control the printer to produce an image calorimetrically matching the scanned image.